1. Field of the Invention
The present invention relates to a charge pump for an integrated circuit.
2. Description of the Related Art
A charge pump is a particular voltage booster circuit, which is used for generating a voltage higher than its power supply voltage. The charge pumps commonly find application in an integrated circuit, for example, comprising a semiconductor non-volatile memory. In fact, the non-volatile memory requires the application of different voltages according to the operation that must be performed (for example, a high voltage is required for its programming and erasing). In order to avoid using a power supply voltage of high value, the integrated circuit includes one or more charge pumps that generate each required voltage from a power supply voltage of lower value.
Operation of a charge pump is based on the continuous accumulation and transfer of electric charge in a series of circuit stages (made by capacitors), which are cascade-connected. In such a way, the voltage at the capacitors increases from an input terminal (that receives the power supply voltage) to an output terminal (that provides the desired voltage). The stages of the charge pump are selectively connected to each other through electronic switches; generally, these switches are implemented by pass-transistors, for example, of the nMOS or pMOS type.
The pass-transistors can be made by forming the various source and drain regions directly in a common substrate of a chip of semiconductor material in which the non-volatile memory is integrated. However, this structure suffers of a body effect. In fact, the substrate must be maintained at a voltage ensuring that the stray diodes formed with the source/drain regions of the first (or of the last) pass-transistor are reverse biased. Consequently, the reverse voltage between the substrate and the source/drain regions of the next (or preceding) pass-transistors will be higher; this involves an increase of the extension of the corresponding depletion region (and therefore of the junction electric field), so that a higher gate voltage will be necessary to obtain the conduction in the respective channel.
The corresponding increase of the threshold voltage of the pass-transistors requires the use of larger components, so as not to degrade their switching efficiency. In any case, this limits the maximum number of stages that can be used for implementing the charge pump.
In order to avoid such problem, it is preferable to make each pass-transistor in a corresponding body region; the body region is insulated from the common substrate directly (if of a different type) or by a further intermediate region otherwise (with a structure known as triple-well). This allows applying power supply voltages of different values to the body regions of the various pass-transistors.
Several biasing circuits of body regions have been proposed for this purpose, so as to remove (or at least reduce) the body effect in the pass-transistors of the charge pump.
For example, document U.S. Pat. No. 6,026,003 proposes using a secondary charge pump for each pass-transistor; the secondary charge pump (which uses diodes as switches) is fed by the voltage that is output by the respective stage. Nevertheless, such structure is rather complex; besides, it is not able to remove the body effect completely (because a reverse voltage different from zero is in any case applied between the body region and the source/drain regions).
A different solution is described in document US-A-2003/0034826. The proposed structure consists of two charge pumps in parallel, which work out-of-phase. Each switch consists of two pass-transistors in series; the intermediate nodes of the two pairs of pass-transistors of each stage are short-circuited to each other. In this structure, each pass-transistor is controlled by the voltage at the adjacent capacitor of the other charge pump. An auxiliary transistor is associated with the first pass-transistor of each charge pump. The body regions of the first pass-transistors of the two charge pumps and of the respective auxiliary transistors are short-circuited to each other. When a generic pair of pass transistors is closed, the corresponding auxiliary transistor equalizes the body region of the first pass-transistor with the voltage at the respective capacitor. At the same time, the pair of pass-transistors in the other charge pump is opened; the intermediate node of such pair of pass-transistors is however maintained at a correct voltage by means of the short-circuit with the intermediate node of the pair of pass-transistors of the other charge pump.
A drawback of the above described structure is that the proposed architecture is not very flexible and it is strictly dependent on the control method of the pass-transistors that is used. Besides, such solution requires the implementation of each switch by two pass-transistors; in fact, the short-circuit between the intermediate nodes of the two pairs of pass-transistors of each stage must be insulated from the next stage (when the first pass-transistors are opened) in order not to interfere with the operation of the charge pump.